A great concept for Riddler OC fanfiction is to make the OC a time - traveler. The OC travels back in time to when the Riddler was just starting out and tries to prevent him from becoming a criminal. This could involve the OC getting involved in the Riddler's past traumas and trying to heal them so that he doesn't turn to a life of crime. It would be a story full of twists and turns as the OC battles against the forces that shaped the Riddler into who he is.
You could write a fanfiction where your Riddler OC is in an alternate universe. For example, in a steampunk world where the Riddler uses elaborate steam - powered contraptions for his riddles and crimes. Your OC could be an inventor who is both fascinated and repelled by the Riddler's actions. Or perhaps a fanfiction where the OC is a journalist trying to expose the Riddler, but ends up getting caught in his web of mysteries.
One idea could be to explore the Riddler's origin story from the perspective of your original character (OC). Maybe your OC is a detective trying to solve the mystery of the Riddler's first crime spree. Another idea is to have your OC be a sidekick of the Riddler, either willingly or by being forced into it. They could then have a complex relationship where the OC tries to influence the Riddler's actions for better or worse.
A prevalent theme is redemption. Sometimes the OC is there to redeem the Riddler from his villainous ways. Also, the theme of identity is common. The OC may be trying to find their own identity while being in the Riddler's world, or they may be helping the Riddler figure out his true self.
There might be a fanfiction where the Riddler somehow gets transported to the Naruto universe and tries to take over using his mind - games. He could start by challenging the village leaders with his riddles, and if they fail, he demands control. This would lead to a series of events where the Naruto characters have to use their unique skills and teamwork to stop him.
You could create an OC for your Persona OC fanfiction who is an artist. This artist OC could use their art as a way to understand and even manipulate the Metaverse. For example, they might draw scenes from the Metaverse that give clues about upcoming events or hidden locations. Their art could also be a form of therapy for other characters who are struggling with their dual lives in the real world and the Metaverse.
Perhaps an OC who is a half - human, half - beast creature. This OC could struggle with acceptance in both the human and beast - kin societies. They end up meeting the main characters during their adventures and learn to embrace their unique identity with the help of the group, while also adding their own combat skills, which are a blend of human strategy and beast - like strength, to the team.
One idea could be an OC who is a brilliant scientist in the Fallout world. They could be working on a secret project to reverse the effects of radiation. Another idea is an OC who is a wandering trader, having all kinds of adventures while traveling from one settlement to another, facing raiders and mutants along the way. And a third idea could be an OC that is part of a new faction trying to bring peace and order to the wasteland.
You could also create an OC human who discovers a hidden talent for communicating with trolls through music. In the fanfiction, this character could use their musical ability to bridge the gap between the two worlds, maybe even creating a new form of harmony between trolls and humans that was previously unthinkable.
One idea could be to create a story where your Bratz OC is a new student in Stilesville. She has a unique fashion sense that both confuses and intrigues the other Bratz girls at first. As the story progresses, they become friends and she helps them design a whole new line of clothing for an upcoming fashion show.
One idea could be creating an OC who is a transfer student with a unique past related to the Dark Hour. For example, they might have had a family member affected by Shadows long before coming to the school. Another idea is an OC with a special ability that doesn't quite fit the normal Persona - using skills that are more about mental manipulation rather than direct combat. And you could also have an OC who is initially a skeptic about the whole Persona thing but slowly gets drawn into it.
One idea could be creating an OC who is a new recruit in the Hellsing Organization. They could have a unique power, like the ability to control shadows, which adds an interesting dynamic to the battles against vampires. Another idea is an OC that is a vampire with a conscience, struggling between their blood - thirsty nature and the human values they still hold. This can create complex moral dilemmas within the Hellsing universe.
A cool Kryptonian OC fanfiction concept could be about a Kryptonian who was exiled from Krypton for a crime they didn't commit. They land on Earth and have to prove their innocence while also dealing with their new life here. Their powers could be fading because of the different environment, and they have to find a way to regain them. Maybe they team up with some of Earth's heroes or anti - heroes to achieve this.